The invention relates to a data transmission system operating on the spread spectrum principle, the system consisting on the one hand of a transmitting portion having a terminal for receiving data to be transmitted at two levels, an adjusting device to have a spreading code word correspond with the first level, this code word being chosen from among the code words D, and the inverted word with the second level, and a transmission circuit for transmitting the output signals of the adjusting device through a transmission channel, and, on the other hand, of a receiving portion, having a receiving circuit for receiving the signals transmitted through the transmission channel and for reproducing, after transmission, the output signals of the adjusting device, a correlation circuit for correlating the reproduced signals and the said code word, the correlation circuit having an input for shift signals which are processed by means of an oscillator circuit, and an output terminal for making the transmitted data available for further processing.
These transmission systems, which operate on the spread spectrum principle, are frequently used to counteract interferences. These systems and their uses are described in particular in the following publications:
the paper by R. C. Dixon entitled: "Spread Spectrum System", published in the collection: A WILEY-INTERSCIENCE PUBLICATION, published by JOHN WILEY AND SONS (NEW YORK . . . )
the article by W. F. UTLAUT entitled: "Principes des techniques d'etalement du spectre--possibilites d'application dans l'utilisation et l'attribution des frequences radio electriques", which appeared in the periodical "LE JOURNAL DES TELECOMMUNICATIONS", Volume 45-1/1978.
In accordance with these systems, for the transmission of data, that is to say a binary element having a given duration, a plurality of binary elements of a shorter duration are transmitted. This plurality of elements corresponds either with the spreading code word for one value of the data, or with the inverted code word for the other value of the data. The choice of the spreading code word is decisive. This code word must have adequate "auto-correlation properties", that is to say the correlation of this word with itself passes through an accurate maximum or minimum for a temporary shift which is equal to zero relative to the other shifts.
French patent specification No. 2,363,268 describes a system which uses two spreading words: these two words are chosen from among the complementary words of the code D. These codes D are particularly defined in the following articles: "Quaternary codes for pulsed radar" by George R. WELTI, published in IRE Transactions on Information Theory, June 1960, and "Complementary Series" by Marcel J. E. Golay, published in IRE Transactions on Information Theory, April 1961. By combining these complementary words a satisfactory auto-correlation function is obtained, as it assumes a zero value for all temporary shifts other than zero. Nevertheless, this described system has the drawback that two transmission channels must be provided for the transmission of each of these two code words, necessitating the use of additional material.